Such a process has been developed in the past by the applicant of the present invention, and reference is made for example to the International Patent application with publication No. WO 99/27036, International Patent application with publication No. WO 03/035803, and International Patent application No. PCT/EP2004/050507 (not published at the priority date of the present application).
WO 99/27036 discloses a process for preparing lower olefins by means of the well-known steam cracking process from a contaminated feedstock. Prior to feeding the feedstock to the steam cracker furnaces the contaminants are removed from the feedstock by means of a membrane separation. By removing contaminants from the feed in this manner it is possible to use, for example, so-called black condensates as feedstock for preparing lower olefins. The term black condensates is commonly used to refer to contaminated natural gas condensates having an ASTM colour of 3 or more. Direct application of these relatively cheap feedstocks in the above steam cracker process would not be possible because the contaminants and/or colour bodies in the feed would give rise to excessive coke formation in convection sections and associated steam cracker furnaces.
The contaminants and/or colour bodies are typically high molecular polynuclear hydrocarbons, which can be present in quantities of several wt % in the hydrocarbon mixture at high colour indices. In testing the process according to WO 99/27036 in a plate-and-frame membrane separation unit it was found that the flux, expressed in feed permeating through the membrane per square meter per day decreased quickly from a maximum value of around for example 1200 kg/(m2·day) to non-economical lower values, and this is attributed to fouling of the membrane surface at the feed side, due to deposition of colour bodies and/or asphalthenes.
International patent application with publication No. WO 03/035803 describes a process to separate colour bodies and/or asphalthenic contaminants from a hydrocarbon mixture by passing part of the hydrocarbon mixture through a membrane over which membrane a pressure difference is maintained thereby obtaining a hydrocarbon permeate having a reduced content of colour bodies and/or contaminants, wherein at selected time intervals the pressure difference over the membrane is substantially lowered by stopping the flow of the hydrocarbon mixture to the feed side of the membrane. Stopping the feed flow can for example be achieved by stopping the operation of a feed pump, or by recycling the hydrocarbon mixture from a position between the feed pump and the membrane to a position upstream of the feed pump. When the membrane operation is resumed after stopping the feed flow, a high permeate flux is observed again.
International patent application No. PCT/EP2004/050507 describes a process for separating colour bodies and/or asphalthenic contaminants from a hydrocarbon mixture using a membrane having a feed side and a permeate side, by contacting the hydrocarbon mixture with the feed side of the membrane, wherein between the feed and permeate sides of the membrane a pressure difference is applied, thereby passing part of the hydrocarbon mixture from the feed side to the permeate side and obtaining at the permeate side of the membrane a hydrocarbon permeate having a reduced content of colour bodies and/or asphalthenic contaminants, and by removing the hydrocarbon permeate from the permeate side of the membrane, wherein during selected time intervals the removal of hydrocarbon permeate from the permeate side of the membrane is stopped so that the pressure difference over the membrane is temporarily substantially lowered. When the membrane operation is resumed again, it was found that permeate can be removed at high flux again.
Therefore, stopping the feed flow as in WO 03/035803, or stopping the permeate flow as in PCT/EP2004/050507, both allowed to operate the membrane separation unit over extended periods of time continuously, without having to replace or take the membrane unit off-line for cleaning.
U.S. Pat. No. 5,785,860 discloses another method for removing asphalthenes from heavy oil, wherein the heavy oil is fed through a ceramic membrane, and wherein the initially large pore size of the ceramic membrane is first reduced by deliberate fouling, following which asphalthenes can be removed for some further time, until the pores are completely blocked.
It is an object of the present invention to provide a membrane separation process for removing colour bodies and/or asphalthenes from a hydrocarbon mixture that allows to operate the membrane unit full-continuous over extended periods of time, and without the need for regular stopping of feed or permeate flux at a time scale of hours.